Republican presidential candidate Donald Trump said on Thursday that the findings of a State Department report about Hillary Clinton's private email server were 'devastating' and indicated behavior that is 'probably illegal'.

But the Republican front-runner stopped short of calling on Clinton to abandon her presidential aspirations over her 'bad judgment'.

'Actually, I sorta like her in the race. I want to run against her,' he told Daily Mail Online during a press conference in North Dakota.

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Donald Trump told Daily Mail Online in a press conference in Bismarck, North Dakota (pictured), that 'there's no reason' as to why Hillary Clinton opened a private email server

'That's gotta be up to her,' he said, 'in terms of whether or not she wants to continue running.'

The report, issued by the State Department's inspector general, found that Clinton's homebrew server, which housed more than 2,000 emails containing material later dubbed 'classified', was the subject of at least two hacking attempts - and Clinton failed to report the incidents.

'Look, she used bad judgment. This was all bad judgment. Probably illegal. We'll have to find out what the FBI says about it. But it certainly was bad judgment,' Trump said of Clinton's actions, which stretched throughout the four years she was secretary of state.

'I just read the report. It's devastating, the report. It's devastating And there's no reason for it,' he said.

'Just, you know, skirting on the edge all the time. And you look back at her history, and this is her history,' he added.

Despite guidelines to the contrary and never seeking approval, Clinton used mobile devices to conduct official business on her personal email account and on that private server, the 78-page analysis concluded.

She never sought approval from senior information officers, who would have refused the request because of security risks, the report said.

And her most senior aides refused to meet with investigators who were looking into why.

Trump said that 'skirting on the edge all the time' has been Clinton's lifelong pattern, but he wouldn't call on Clinton to get out of the presidential race

Trump's press conference was his first since the night of May 3, when took questions night from reporters at Trump Tower in New York, following his victory in the Indiana primary and Texas Sen Ted Cruz's announcement that he was quitting the presidential race.

It was also his first moment meeting the press since the Associated press declared that he had cemented the Republican presidential nomination - with the help of North Dakotans who gave him a majority.

'The folks behind me got us right over the top,' he said.

He fielded questions from reporters in front of a group of 20 state lawmakers, most of whom will be delegates to the Republican National Convention.

One, John Trandem, said he was Trump's 1,238th pledged delegate - the one who put him over the top and cemented the presidential nomination.

Ben Koppelman, a state senator standing behind him, joked that he and Trandem were on the phone accepting their slots in the GOP convention delegation at the same time, so he might have been the clincher

Trump was in Bismarck to deliver a rally speech at the Bismarck Event Center on the sidelines of the Williston Basin Petroleum Conference, an annual convention of oil and gas magnates.

He was expected to deliver the latest in a series of pronouncements dismissing global warming as a hoax, in a part of the United States where fossil fuels provide the rich center of the economy.

Trump said recently that he is 'not a big fan' of the Paris climate agreement, which President Barack Obama has committed the United States to honoring.

'I will be renegotiating those agreements, at a minimum, and at a maximum I may do something else,' the billionaire told Reuters last week, flying in the face of a consensus process between wealthy and emerging countries from which the United States could not easily withdraw.